Investigation of the Effect of Bond-Slip on Cracking of RC Beams Utilizing Lattice Models

Cracking analysis of reinforced concrete structures is a necessary stage in their design. Determing the width of cracks and crack length is generally an empirical argument; however, analytical relations have been introduced for this purpose. Despite numerous studies in this area, provided relationships in estimating the distance of the cracks, the crack width and crack propagation mode in most situations seem insufficient. The purpose of this study is to investigate the effect of different models bond-slip behavior of reinforced concrete beams, with or without transverse bars, and also large-scale analysis is used. Bond between concrete and steel on the performance and lifetime of structures under the loading have a significant impact and weak bond can cause problems in force transfer mechanism, resulting in the increase of deformation, number of cracks, cracks width and reduces the bearing capacity of reinforced concrete members. These factors, in turn, can lead to sudden failure of members. Therefore, predicting the behavior of bond-slip in reinforced concrete structures is essential. Previously, many experimental works have been done to evaluate and understand how the crack propagation ocurrs. Most of these research works were done on the bond behavior of concrete specimens with steel rods buried in them. Methodsproposed by researchers are only applicable to small samples and are not suitable for reinforced concrete structures. Few studies have been done to confirm and refine bond-slip models (which are obtained experimentally); however, it seems that this issue requires further studies. In the present study, four bond-slip models (CEB-FIP, Farra, Den and Bigaj, and Haskett) in numerical analysis are applied to study their effect on the amount of crack width, crack spacing and crack propagation in reinforced concrete members. In order to show the effect of bond-slip on the performance of reinforced concrete structures, tested beam by Leonhardt [1] with the same geometry and material has been modeled in ABAQUS software [2] and crack propagation has been investigated, using nonlinear analysis and numerical lattice model. The bond-link element proposed by Ngo and Scordelis [3] has been used to model the bond between concrete and steel. In the past 50 years, lattice models have been used for different purposes and in different species. One recent study by Kim et al. [4] was carried out in which the fracture behavior of concrete structures was simulated using random lattice model with four-point bending test with high speed loading. In this research, a lattice model based on frame method which was proposed by Hrennikoff [5] was used. In this way, a structure was modeled with pattern of similar truss members (Fig. 1), cracking process and its propagation was shown by the failure of the concrete bar elements. According to the suggestion of Chen and Baker [6], lengths of vertical and horizontal members were equal and cross-section lateral members and diagonal members were considered equal. Each truss element consists of a two point of same levels in two directions which are perpendicular and parallel to the bar which has a nodal members resist against shear stress. Thus, this model is expected to estimate both flexure and shear failures and shear carrying capacity of reinforced concrete members with acceptable accuracy. In addition, because the lattice model can divide a concrete member into the truss elements, the internal stresses can be easily followed. In Farra model, middle flexural cracks have the minimum height and in Haskett model they have maximum height compared to the other models. The damage of Den and Bigaj model which is created in the top of the beam is greater than the other models. In all four models, the shear cracks begin almost together, but their propagations are different. In the Farra model, shear cracks are not continuous and stress concentration is lower than other models and also in the Farra model, shear crack angle to the horizon is the highest and in the CEB-FIP model, it is the lowest. In the Farra model, flexural cracks thickness is less than other models and the Haskett model, flexural crack thickness is greater than the other models. In the Den and Bigaj model, crack propagation along the bar is high and in the CEB-FIP model it is less than the other models. In general, in terms of crack propagation, three models of Haskett, Den and Bigaj and CEB-FIP are much closer to each other. Also results show that different models of bond-slip can affect the crack propagation.In all bond-slip models, the flexural failure is ocurrs in the beam. CEB-FIP and Haskett models are nearly the same, while in Den and Bigaj and Farra models, small cracks are seen in the middle part. In Den and Bigaj model, unlike the rest of the models, crack of concrete cover was not created in the center of the beam and also in the CEBFIP and Hasket model, cracks in concrete cover center was propagated to the top of the beam's cross section. In Farra and Den and Bigaj models, major cracks are almost straight, but in the other two models they are curved. In general, the propagation pattern of cracks with shear bars in Haskett and CEB-FIP models better simulated compared to the other two models. The main scope of the present study includes the assessment of cracked reinforced concrete beam with a large scale lattice model based on fracture mechanics and the evaluation of four different models of bond-slip. According to the findings of this study, the following conclusions can be attained: - The results show that the analytical model used are able to reproduce the most important aspects of crackpropagation in reinforced concrete members under monotonic loading. - Lattice model can limit the shear cracks in the reinforcement beam with shear bars properly. - Bond-link element defined by minimum energy method is able to reflect the effect of bond-slip in the distance of cracks as the concrete between the cracks will be unloaded in the final pattern of cracks. - In the four models studied, the effect of bond-slip was evident in the form of localized cracks in concrete around the bars. Overall, the CEB-FIP model can show crack propagation better than other models both in shear and flexural cracks.- An important feature of this model is the ability to show the crack orientation by the element failure. It can also specify the initial cracks with element removing.